The present invention relates to the hot deformation of a work piece, in particular by folding, so that this piece has a final shape different from the initial shape. More particularly, the invention is applicable to such deformation of work pieces of plastics material.
In this technical field, difficulties have been encountered as concerns repeatability and regularity in the obtainment of identical pieces, in particular as concerns the amplitude of the deformation, for example the folding angles, and the final appearance of the material in the deformed zone, this appearance depending not only on the degree of deformation of the material but also on the temperature to which the latter is brought. At the present time, to effect a hot deformation of a work piece, in particular a piece of plastics material, the material is heated in the zone to be deformed, and the characteristics of this heating, i.e. its duration, the temperature of the source of heat, the intended temperature of the material in its deformation zone, the precision of the heat flow, and other parameters, are controlled and monitored in such manner that, finally, the piece is heated for a predetermined period of time, after which, possibly after having removed the source of heat, the piece is mechanically deformed with a suitable tool.
However, it has been found that this calculated predetermined duration of the heating must be modified in accordance with the ambient temperature; this is why there are associated with the known installations thermal sensors which detect the variations in the ambient temperature and correct, possibly automatically, this heating duration by shortening or lengthening it.
Notwithstanding this precaution, the presently-known method for the thermal deformation of a work piece, in particular a piece of plastics material, presents serious drawbacks which are related to the fact that the unmeasured variations of certain parameters are not taken into account, for example the initial temperature of the work piece to be deformed, the temperature of certain mechanical elements cooperating with the work piece, for example the support or the guides for the work piece, or the deforming tool, and the stability of the source of heat.
Consequently, even if the predetermined heating duration is conformed to, it often occurs that, in the course of the piece deforming stage proper, the zone to be deformed is not at the required temperature, either because the temperature is too low, which is liable to result in mechanical degradation or fracture of the work piece, or because the temperature is too high, which has for consequence for example thermal degradation by modification of the structure of the work piece.
Another method (British patent 898 095) comprises first of all forcefully deforming the work piece, optionally after a preheating, so as to impart the desired final shape thereto, then subjecting the piece, in this deformed state, to a heating so that the initial deformation is maintained. However, in this method, on one hand, the deformation zone of the piece is subjected to large mechanical stresses which are liable to cause its degradation and even its fracture and, on the other hand, the operational conditions of the heat treatment are just as uncertain as in the first aforementioned method.